FIG. 1 illustrates a typical wind turbine 1. The wind turbine comprises a wind turbine tower 2 on which a wind turbine nacelle 3 is mounted. A wind turbine rotor 4 comprising at least one wind turbine blade 5 is mounted on a hub 6. The hub 6 is connected to nacelle 3 through a low speed shaft (not shown) extending from the nacelle front. The wind turbine illustrated in FIG. 1 may be a small model intended for domestic or light utility usage, or may be a large model, such as those that are used in large scale electricity generation or on a wind farm for example. In the latter case, the diameter of the rotor could be as large as 100 meters or more.
In relation to a first aspect of the invention, as the wind turbine is subjected to impulses from the wind, or other weather conditions, oscillations can be established in the rotor blades, the vibrations being into and out of the plane of rotation of the rotor. It is desirable to monitor these vibrations since excessive vibration can cause damage to the blades, the rotor hub or other structural components.
Existing methods of measuring such vibrations rely on accelerometers or similar such devices mounted directly on or within the blade, or on the wind turbine hub. Such arrangements are difficult to maintain because of the difficulty in accessing the sensors, and the difficulty in providing communication paths for the sensors to return data to a central processing unit.
We have appreciated that it would be desirable to provide an alternative system and method for detecting blade vibrations.
In relation to embodiments of a second aspect of the invention, radar systems for the avoidance of collision between an aircraft and a ground-based obstacle have previously been proposed. For example, EP-A-1,486,798 describes an obstacle collision avoidance system (OCAS) that comprises a radar system that continuously scans a coverage area for moving aircraft. Upon detection of an aircraft within the coverage area, the radar is adapted to follow the aircraft as a defined target. The course, height and speed of the target are computed and if the parameters are such that there is a danger of collision of the aircraft with the obstacle, the system is activated to issue a warning signal to the pilot of the oncoming aircraft.
The OCAS system described in EP-A-1,486,798 includes one or more field units which are mounted on, or adjacent to the obstacle or obstacles of which warning is to be given. Each field unit incorporates the radar system described above and one or more warning devices for issuing warning signals to oncoming aircraft. A central processing unit processes the information from the radar system and controls the warning devices based on the processed information. In preferred embodiments, the warning device comprises a warning light that is switched on upon detection of an aircraft. In addition or as an alternative to the warning light, the field unit may comprise an audio warning device, such as a radio, for emitting audio warning signals to the pilot. The warning devices are operated in order to activate a warning in good time before a potential collision, such that the pilot has sufficient time to notice the obstacle and take evasive action to manoeuvre away from the obstacle.
Each field unit of the obstacle collision avoidance system of a wind farm is preferably equipped with a radar antenna as described in ER-A-1,486,793, which enables the desired scanning to be performed, as described above. The radar antenna preferably comprises a plurality of antenna panels mounted on a framework such that the panels face outwards from the framework in different directions. Particularly preferably, the antenna panels are arranged as the sides of an octagon.
Each antenna panel comprises a plurality of antenna elements which are selected according to frequency, vertical coverage requirements etc. With the arrangement of antenna panels described above, the antenna elements form an array that extends along both a horizontal (azimuth) axis and a vertical (elevation) axis to provide the desired coverage of the radar. A suitable radar antenna arrangement for use in the field units of the present invention is described in WO-A-2011/085237.
We have appreciated that it would be desirable to be able to perform diagnostic checks on an array radar system such as the sort used in the OCAS arrangement.